1. Field of the Invention
The present invention relates to an image pickup apparatus having an optical member between an image pickup optical system and an image sensor.
2. Description of the Related Art
FIG. 1 schematically shows the internal structure of a digital single lens reflex camera as an example of a conventional image pickup apparatus.
In FIG. 1, the digital single lens reflex camera (hereinafter referred to as “DSLR”) includes a replaceable photographing lens 1, a reflecting mirror 2, a focal-plane shutter 3, an optical filter 4, an image pickup unit 5, and a finder optical system 6. The reflecting mirror 2 can swing so as to move out of the optical path of the photographing optical system during photographing. The focal-plane shutter 3 mechanically controls the exposure time. The optical filter 4 includes an optical low-pass filter (to be hereinafter described) and a luminosity correcting filter attached to each other. The image pickup unit 5 is disposed at a predetermined focal point of the photographing lens 1 and includes a solid-state image sensor (e.g., a CCD (Charge-Coupled Device) or a CMOS (Complementary MOS)). The finder optical system 6 is disposed above the reflecting mirror 2. After passing through the photographing lens 1, light is led to the finder optical system 6 by the reflecting mirror 2.
The reflecting mirror 2 is located in the optical path of the photographing optical system. When photographing is started by pressing down a release switch (not shown), the reflecting mirror 2 moves out of the optical path of the photographing optical system.
When the reflecting mirror 2 is located in the optical path of the photographing optical system, a subject image formed by the photographing lens 1 can be observed as a finder image through the finder optical system 6. When the reflecting mirror 2 is out of the optical path of the photographing optical system, by releasing the focal-plane shutter 3, the subject image is formed on an image pickup surface of the image pickup unit 5 through the optical filter 4. The image pickup unit 5 outputs an image pickup signal, which is stored in a memory (not shown).
When a subject has high-frequency components, a solid-state image sensor (e.g., a CCD or a CMOS) can generate false signals depending on its sampling frequency. An optical low-pass filter (hereinafter referred to as “optical LPF”) can be used for reducing such false signals.
Such an optical LPF includes a combination of one or more quartz birefringence plates and one or more quartz phase plates. In general, a first quartz birefringence plate, a quartz phase plate, and a second quartz birefringence plate can be arranged in this order.
The optical axis of the first quartz birefringence plate is tilted at a predetermined angel, typically at an angle of 45 degrees, with respect to a normal to the entrance plane of the plate. The first quartz birefringence plate is disposed such that the orthogonal projection of its optical axis onto the entrance plane of the plate is in the horizontal direction of the image sensor so as to lower the spatial frequency response. In addition, the first quartz birefringence plate has functions of separating incident light into linearly polarized ordinary and extraordinary rays having predetermined planes of vibration, and of moving the extraordinary ray in the horizontal direction of the image sensor by a predetermined distance.
The optical axis of the quartz phase plate is perpendicular to a normal to the entrance plane of the plate, that is to say, in the plane of the plate. The quartz phase plate is disposed such that its optical axis is at an angle of 45 degrees with respect to the orthogonal projection of the optical axis of the first quartz birefringence plate onto the entrance plane of the plate, that is to say, at an angle of 45 degrees with respect to the horizontal direction. In addition, the quartz phase plate has a function of changing the phases of the linearly polarized ordinary and extraordinary rays having predetermined planes of vibration output from the first quartz birefringence plate. The quartz phase plate changes the phases of the incident rays by a phase difference determined by the thickness of the quartz phase plate and the wavelength of the incident rays.
The optical axis of the second quartz birefringence plate is tilted at a predetermined angel, typically at an angle of 45 degrees, with respect to a normal to the entrance plane of the plate. The second quartz birefringence plate is disposed such that the orthogonal projection of its optical axis onto the entrance plane of the plate is in the vertical direction of the image sensor so as to lower the spatial frequency response. In addition, the second quartz birefringence plate has functions of separating incident light into linearly polarized ordinary and extraordinary rays having predetermined planes of vibration, and of moving the extraordinary ray in the vertical direction of the image sensor by a predetermined distance.
In the above-described example, by combining and arranging the first quartz birefringence plate, the quartz phase plate, and the second quartz birefringence plate with the directions of their optical axes appropriately set, the subject image is separated in the horizontal and vertical directions of the image sensor into four images in total, thereby lowering the spatial frequency response in the horizontal and vertical directions of the image sensor corresponding to the separation distance.
However, if a replaceable lens designed for a silver salt film camera is used as a photographing lens for a DSLR such as one shown in FIG. 1, the following difficulties can arise. First, a significant change in the ray aberration of the photographing lens 1 can occur due to the interaction of the ray with the optical filter 4, disposed between the photographing lens 1 and the image pickup surface, and a cover glass, attached to the image pickup unit 5 for protecting the image sensor. In addition, the space between the focal-plane shutter 3 and the image pickup surface for disposing the optical filter 4 will be very small.
To alleviate the above difficulties, the following proposals have been made.
A first conventional technology uses a plate of lithium niobate. The separation distance between ordinary and extraordinary rays is determined by (1) the ordinary and extraordinary refractive indices of a single crystal material, (2) the angle between a normal to the entrance plane of a single crystal plate and its crystal axis, and (3) the thickness of the plate. On the premise that (2) the angle between a normal to the entrance plane of a single crystal plate and its crystal axis is efficiently set, the separation distance between ordinary and extraordinary rays is determined generally by (1) the ordinary and extraordinary refractive indices of a single crystal material. The difference between ordinary and extraordinary refractive indices of lithium niobate is bigger than that of quartz. Therefore, when the separation distance between ordinary and extraordinary rays is the same, an optical LPF using a lithium niobate plate can be thinner than an optical LPF using a quartz plate.
A second conventional technology uses a liquid crystal optical filter such that the liquid crystal is injected between two substrates opposite to each other (for example, Japanese Patent Laid-Open No. 6-317765). By providing the liquid crystal molecules with a particular alignment, the filter has a large birefringent anisotropy. Therefore, reduction in the size, thickness, and cost of an optical LPF, which is not expected in an optical LPF using a quartz plate, can be achieved.
A third conventional technology uses a polymeric optical LPF (for example, Japanese Patent Laid-Open No. 8-122708). A particular photo-polymerizable liquid crystal composition is photo-polymerized, and the obtained optically anisotropic polymeric film is used as an optical LPF. Compared to quartz, a very large anisotropy of refractive index can be obtained. Therefore, reduction in the size, thickness, and weight of an optical LPF can be achieved.
A fourth conventional technology uses an optical LPF formed of a birefringent material as a cover for protecting an image sensor (for example, Japanese Patent Laid-Open No. 2000-114502). Since a conventional cover glass can be omitted, the thickness of the whole optical plate can be reduced by the thickness of the conventional cover glass.
However, the first to fourth conventional technologies can have the following difficulties.
In the first conventional technology, by forming an optical LPF of a single crystal material of lithium niobate, the whole optical LPF can be thin. However, it can be difficult to polish the plate due to the hardness of the material itself. In addition, since the material has a high refractive index, a high level of antireflective process can be necessary, and therefore the cost increases.
In the second and third conventional technologies, attention is focused on properties of a certain kind of organic material used for liquid crystal and so on, such that the alignment direction of molecules of the organic material is changed by applying an electric field or a magnetic field. Characteristics such that when the alignment direction is uniform, the organic material exhibits an anisotropy related to that of single crystal materials, are applied to an optical LPF. However, it can be difficult to make the alignment direction uniform. Whatever methods are used, alignment nonuniformity remains, and consequently light scattering occurs. A drawback of light scattering is, for example, the fact that, in the case of a point light source, a blur halo can be generated around it to deteriorate the image quality. If liquid crystal layers are layered in the second conventional technology, or if optically anisotropic polymeric films are layered in the third conventional technology, this light scattering is increased approximately in proportion to the number of the layers.
In the fourth conventional technology, since a conventional cover glass for protecting an image sensor can be omitted from the image pickup unit, the thickness of the whole optical plate can be reduced, the spatial efficiency can be improved, and the ray aberration can be reduced. However, in order to be used as a cover glass, the optical LPF needs to be larger than an ordinary optical LPF. In addition, since the optical LPF is close to the image pickup surface, defect control of quartz needs to be stricter. Thus, the cost can be high compared to the case where a conventional cover glass is used.